1. Field of the Invention
The present invention relates to a method and system for Multicast Broadcast Service (MBS) over Mobile Multi-hop Relay (MMR) network using dynamic modification of Modulation and Coding Scheme (MCS) level. More particularly, the present invention relates to a method and system for MBS over MMR network using dynamic modification of MCS level that offers high quality services on mobile WiMAX by dynamically modifying the MCS level for the transmission between Relay Station (RS) and mobile station (MS) based on the MCS level for the transmission between the Base Station (BS) and the RS which can be periodically updated.
2. Description of the Related Art
Recently, demands for various broadcast service have been increasing in wireless communication environment. In order to provide high quality broadcast service such as live broadcast, hook-up, real-time video service, etc., reliability and high data throughput are required and thus it is necessary for wireless communication environment to have enough system resources such as bandwidth as wired communication environment has. However, wireless communication environment such as mobile WiMAX uses expensive wireless system resources and thus services provided by such wireless communication environment are limited.
MBS based on IEEE802.16e standard provides service that can overcome these problems. The MBS is introduced to make many mobile users effectively use limited systems sources through shared channel. However, the MBS has disadvantages that it is possible to not provide enough quality service to the mobile stations that are located far from the base station or particularly at the edge of the cell. It is because, as the distance from the base station gets bigger, Carrier to Interference and Noise Ratio (CINR) gets lower and low data throughput due to the lowered CINR lowers the service quality. That is, MBS data throughput significantly depends on the channel condition. The existing MBS system provides service based on the lowest CINR measured between the base station and the mobile stations so that the throughput is limited. Accordingly, it is noted that, although the MBS is useful technology capable of simultaneously transmitting data to many mobile users by effectively taking advantage of the limited system resources, the MBS has disadvantages that it is difficult to provide stable service quality with enough data throughput to all mobile users.
Mobile WiMAX system for supporting MBS needs to include MBS controller for controlling various information and MBS contents server transmitting encoded MBS contents data, etc. Also, it is necessary to include MBS zone, which is a group of base stations serving synchronized MBS contents data to mobile stations. The MBS zone supports the MBS system by being operated with the single frequency. It is also obvious to people skilled in the art that multiple MBS zones are also feasible.
Mobile WiMAX system can transmit data to a target mobile station using a 16-bit connection ID (CID) in MAC layer. It also supports macro diversity, which improves the probability of successful data reception and reduces power consumption and overall interference.
Recently, many studies to solve problems of service quality deterioration due to low data throughput are under progress by IEEE802.16j project group. The IEEE802.16j project group considers using the MMR technology in the IEEE802.16 standard-based system to expand the service coverage and improve data throughput. Relaying technology to maintain the existing point-to-multipoint (PMP) frame structure defined in IEEE802.16e standard is being studied for the expansion of the service coverage through relay stations and another technology to effectively use the MCS level is being studied for the enhancement of the data throughput.
In the IEEE802.16j MMR network, there are three types of relaying concept such as fixed relay station (FRS), nomadic relay station (NRS), and mobile relay station (MRS). First, the FRS is installed and used in a fixed location where reception from the base station is difficult or reception distance should be expanded. Second, the NRS is installed and used in a place where mobile users temporarily gather, in order to prevent temporary load on the system. Third, the MRS is installed and used in mobile transportations such as bus and train, etc. to provide stable service to mobile users on the move. Due to the introduction of such relay systems, it is possible to offer service with guaranteed quality to mobile users regardless of time and place.
Adaptive Modulation and Coding (AMC) scheme is to selectively choose the MCS level appropriate for the current channel condition and the employment of the AMC scheme significantly improves performance of the overall system. The AMC scheme is also to selectively choose the appropriate MCS level considering the average channel condition of the all users. The AMC scheme is developed to guarantee high spectrum efficiency despite variation of channel conditions and it is one of important standard technologies of the wireless communication system. The main feature of the AMC scheme is to dynamically select the MCS level for frames to be able to adaptively modify the overall spectrum efficiency according to the channel conditions. In connection with this, the mobile stations transmit feedback information to the base station in response to the frames transmitted through the channel and thus it is possible to decide the appropriate MCS level.
The mobile WiMAX system supports various MCS levels which can be modified depending on the channel conditions. The mobile stations provide the feedback information regarding quality of the downlink channel to the base station through channel quality indicator (CQI). In order to check quality of the uplink channel, the base station checks the channel condition through the signal that the base station receives. Then, AMC scheduler decides appropriate MCS level based on the quality of the uplink channel and the downlink channel. In the downlink mode, the Mobile WiMAX system uses three types of modulation: Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (QAM), and 64QAM. The modulation scheme, combined with the coding scheme, is used as Modulation and Coding Scheme levels. With suitable modification of the MCS levels depending on the channel conditions, it becomes possible to achieve high data throughput.
In the mobile WiMAX system, mobile stations in the same location use the identical MCS level and the low order MCS level is used as the channel condition gets worse due to the increase of the distance from the base station.
FIG. 1 illustrates application of different MCS levels depending on locations of MSs.
Channel conditions are reported through channel quality indicator channel (CQICH) from mobile stations so that reception condition of the mobile stations can be checked. The CQICH is allocated to the mobile stations using the control IE. It is used to report the downlink CINR for either diversity sub-channels or band AMC sub-channels.
According to AMC scheme in the mobile WiMAX system, the receiver side asks for MCS level with which it wishes to receive MBS data based on the MCS level and the CINR of the current channel. However, it is impossible to find the most adequate MCS level only with the measurement of the CINR of the current channel. It is because, according to the AMC scheme, it is hard to determine the adequate MCS level with one step while it is possible to relatively higher or lower the MCS level based on the measured CINR. Further, it is not possible to provide stable service in an environment where a number of mobile stations simultaneously receive MBS data.